One factor USA intelligence is trying to determine is the role the Russians are playing in this effort - chemical weapons use has been condemned by the Kremlin, but if Assad proceeds, it may demonstrate he is trying to publicly show independence from Moscow, officials say.
The White House said on Monday it appeared the Syrian government was preparing for another chemical weapons attack and it warned Assad that he and his military would "pay a heavy price" if it went ahead.
The White House issued the warning on Monday to Syria's leadership based on intelligence about what appeared to be active preparations at a Syrian airfield allegedly used for such an attack in April, US officials said on Tuesday.
Pentagon spokesman Navy Captain Jeff Davis said that the U.S. had seen "activity" at Shayrat airfield that showed "active preparations for chemical weapons use".
USA warships launched 59 cruise missiles at a Syrian airbase in early April in response to a chemical attack believed to be carried out by forces loyal to Syrian President Bashar Assad.
Russian officials have described the war in Syria as the biggest source of tension between Moscow and Washington and say the April cruise missile strike ordered by U.S. President Donald Trump raised the risk of confrontation between them.
At the time Syrian President Beshar al-Assad dismissed allegations of the chemical attack as "100% fabrication".
Iran's Foreign Ministry spokesman Bahram Qassemi also warned against a "possible unilateral and illegal" military moves by the USA under "hollow pretexts", saying such acts would only lead to "chaos and more insecurity" in the region and give new life to "failing terrorists".
"The activities are similar to preparations the regime made before its April 4, 2017 chemical weapons attack".
"The goal is at this point not just to send Assad a message but to send Russian Federation and Iran a message that if this happens again, we are putting you on notice", Haley said. A senior Russian lawmaker accused the US of a "provocation".
Asked if Russia had warned the U.S. administration against any unilateral action in Syria, Russia's deputy foreign minister, Gennady Gatilov, replied that Russian officials have "always spoken about that, including in relation to their [US] latest strikes on Syrian armed forces".
The statement, which came with little explanation, was a warning shot to both Assad and the Russian leaders who are backing his regime.
The Kremlin was not happy with the White House statement.
A Pentagon spokesperson later confirmed the alleged preparations were being made at the same airfield the USA struck in April.
The State Department officials weren't authorized to discuss national security planning publicly and requested anonymity.
The White House threat caught many in Trump's administration by surprise.
White House spokeswoman Sarah Huckabee Sanders flatly denied the White House was freelancing on foreign policy.
Syria maintained it hadn't used chemical weapons and blamed opposition fighters for stockpiling the chemicals.